U.S. patent application number 09/212405 was filed with the patent office on 2001-05-24 for electrode for an electric double layer capacitor and process for producing it.
Invention is credited to ISHIKAWA, TAKAMICHI, KUROKI, SATORU, SUHARA, MANABU.
Application Number | 20010001590 09/212405 |
Document ID | / |
Family ID | 18444759 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001590 |
Kind Code |
A1 |
ISHIKAWA, TAKAMICHI ; et
al. |
May 24, 2001 |
ELECTRODE FOR AN ELECTRIC DOUBLE LAYER CAPACITOR AND PROCESS FOR
PRODUCING IT
Abstract
A process for producing an electrode for an electric double
layer capacitor, which comprises extruding a mixture comprising a
carbonaceous material, a polytetrafluoroethylene and a processing
aid by paste extrusion, and rolling the obtained extruded product
by rolling rolls to form it into a sheet shape.
Inventors: |
ISHIKAWA, TAKAMICHI;
(KAWASAKI-SHI, JP) ; KUROKI, SATORU;
(KAWASAKI-SHI, JP) ; SUHARA, MANABU;
(YOKOHAMA-SHI, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
18444759 |
Appl. No.: |
09/212405 |
Filed: |
December 16, 1998 |
Current U.S.
Class: |
361/502 |
Current CPC
Class: |
H01G 11/72 20130101;
H01G 9/155 20130101; Y02E 60/13 20130101; H01G 11/32 20130101; H01G
11/38 20130101; H01G 11/86 20130101 |
Class at
Publication: |
361/502 |
International
Class: |
H01G 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 1997 |
JP |
9-355589 |
Claims
What is claimed is:
1. A process for producing an electrode for an electric double
layer capacitor, which comprises extruding a mixture comprising a
carbonaceous material, a polytetrafluoroethylene and a processing
aid by paste extrusion, and rolling the obtained extruded product
by rolling rolls to form it into a sheet shape.
2. A process for producing an electrode for an electric double
layer capacitor, which comprises preliminarily molding a mixture
comprising a carbonaceous material, a polytetrafluoroethylene and a
processing aid, then intermittently and sequentially supplying it
to a cylinder mold for extrusion molding to continuously extrude it
by paste extrusion to obtain an elongate extruded product, and
rolling the obtained extruded product by rolling rolls to form it
into a sheet shape.
3. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein the mixture contains
from 1 to 50 wt % of the polytetrafluoroethylene and from 20 to 200
wt % of the processing aid, based on the carbonaceous material.
4. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein the extrusion drawing
ratio in the paste extrusion is from 5 to 500.
5. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein the temperature of
the rolling rolls is from 20 to 350.degree. C.
6. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein after rolling the
extruded product by rolling rolls to form it into a sheet shape,
the formed product of sheet shape is monoaxially or multiaxially
stretched from 1.1 to 5.0 times the original length and further
rolled by rolling rolls.
7. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein the carbonaceous
material comprises an activated carbon having a specific surface
area of from 200 to 3,500 m.sup.2/g and a particle size of from 0.1
to 100 .mu.m and a carbon black having a specific surface area of
from 200 to 1,500 m.sup.2/g and a particle size of from 0.001 to 1
.mu.m.
8. The process for producing an electrode for an electric double
layer capacitor according to claim 1, wherein after rolling the
extruded product by rolling rolls to form it into a sheet shape,
the formed product of sheet shape is dried to remove the processing
aid.
9. An electrode for an electric double layer capacitor, which is a
formed product of sheet shape prepared by forming a carbonaceous
material by means of a polytetrafluoroethylene as a binder and
which has a thickness of from 0.005 to 0.25 mm, a porosity of from
50 to 80% and the tensile strength in one direction is at least 1.5
kg/cm.sup.2.
10. The electrode for an electric double layer capacitor according
to claim 9, which is prepared by extruding a mixture comprising the
carbonaceous material, the polytetrafluoroethylene and a processing
aid by paste extrusion, and rolling the obtained extruded product
by rolling rolls to form it into a sheet shape.
11. The electrode for an electric double layer capacitor according
to claim 10, wherein the tensile strength in the paste extrusion
direction is at least 1.5 kg/cm.sup.2.
12. The electrode for an electric double layer capacitor according
to claim 9, wherein the polytetrafluoroethylene is formed into
fibers to have a three-dimensional network structure.
13. The electrode for an electric double layer capacitor according
to claim 9, wherein from 1 to 50 wt. % of the
polytetrafluoroethylene is contained, based on the carbonaceous
material.
14. The electrode for an electric double layer capacitor according
to claim 9, wherein the carbonaceous material comprises an
activated carbon having a specific surface area of from 200 to
3,500 m.sup.2/g and a particle size of from 0.1 to 100 .mu.m and a
carbon black having a specific surface area of from 200 to 1,500
m.sup.2/g and a particle size of from 0.001 to 1 .mu.m.
15. An electric double layer capacitor, which comprises an
electrode made of a formed product of sheet shape prepared by
forming a carbonaceous material by means of a
polytetrafluoroethylene as a binder and which has a thickness of
from 0.005 to 0.25 mm, a porosity of from 50 to 80% and the tensile
strength in one direction is at least 1.5 kg/cm.sup.2.
16. The electric double layer capacitor according to claim 15,
wherein the formed product of sheet shape is prepared by extruding
a mixture comprising a carbonaceous material, a
polytetrafluoroethylene and a processing aid by paste extrusion,
and rolling the obtained extruded product by rolling rolls to form
it into a sheet shape.
17. The electric double layer capacitor according to claim 16,
wherein the formed product of sheet shape has a tensile strength in
the paste extrusion direction of at least 1.5 kg/cm.sup.2.
18. An electric double layer capacitor, which comprises an
electrode which is prepared by preliminarily molding a mixture
comprising a carbonaceous material, a polytetrafluoroethylene and a
processing aid, then intermittently and sequentially supplying it
to a cylinder mold for extrusion molding to continuously extrude it
by pate extrusion to obtain an elongate extruded product, and
rolling the obtained extruded product by rolling rolls to form it
into a sheet shape.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electrode for an
electric double layer capacitor and a process for producing it.
[0003] 2. Discussion of Background
[0004] The demand for electric double layer capacitors utilizing an
electric double layer formed at the interface between a polarizable
electrode and an electrolyte, particularly coin shaped ones, is
rapidly increasing recently, as a memory backup power. On the other
hand, the development of an electric double layer capacitor having
a large capacitance per volume, small internal resistance, a high
energy density and a high output density, is required, for a use
wherein a large capacity is required, e.g. as a driving source for
electric cars. Further, with regard to an electric double layer
capacitor for memory backup, it is desired to reduce the internal
resistance.
[0005] The electrode for an electric double layer capacitor is
prepared, for example, by kneading an activated carbon powder with
a solvent of an electrolytic solution such as sulfuric acid, and
forming the mixture into a slurry, followed by press forming (U.S.
Pat. No. 3,288,641). However, the electrode obtained by the process
has a rigid porous structure and thus is likely to be cracked or
broken, and it can not be used for a long period of time. On the
other hand, a carbon-based electrode has been proposed which is
made of a viscous material having a binder such as a
polytetrafluoroethylene (hereinafter referred to as PTFE) added to
a mixture comprising an activated carbon powder and an electrolytic
solution, as the case requires (JP-B-53-7025, JP-B-55-41015). The
electrode has crack resistance and break resistance. However, it is
inadequate in the shape keeping property, and a cell having a
special structure is required to supplement the strength to use the
electrode.
[0006] In order to obtain an electrode having crack resistance and
break resistance, and an excellent shape keeping property, a
process has been proposed, which comprises preliminarily molding a
kneaded material comprising a carbonaceous material, a binder such
as PTFE and a liquid lubricant, followed by stretching or rolling
to obtain a formed electrode of sheet shape (JP-A-63-107011,
JP-A-2-235320). However, by this process, PTFE is randomly formed
into fibers by kneading, and a part of PTFE is formed into fibers
and the rest is not, and consequently the hardness of the two parts
will be different. Therefore, when forming the sheet electrode into
a thin film sheet having, for example, a thickness of at most 0.2
mm, the surface tends to be irregular, and holes are likely to be
formed. Therefore, the capacitance per volume (hereinafter referred
to as capacitance density) of the electric double layer capacitor
can not be made large, and the internal resistance is large.
[0007] A process has also been proposed, which comprises mixing an
activated carbon powder and PTFE to obtain a paste, coating the
paste on a current collector, followed by drying, heating at a
temperature higher than the melting point of PTFE, and
press-forming the electrode to make it thin to increase the density
(JP-A-9-36005). However, with this process, the production steps
are complicated, and it is difficult to continuously conduct the
process, and a part of PTFE melts so that the internal resistance
will be high.
[0008] On the other hand, for extrusion of PTFE, a paste extrusion
method has been known, which comprises using a polymer (fine
powder) obtained by coagulating and drying an aqueous dispersion of
PTFE made by emulsion polymerization of a tetrafluoroethylene,
adding a processing aid such as naphtha or white lamp oil thereto,
preliminarily molding the mixture to form it into a sleeve shape
and filling it to a cylinder mold, followed by pressurizing by a
ram to extrude it through a nozzle suitable for a shape of a rod or
a sheet, and vaporizing the processing aid to obtain a formed
product (U.S. Pat. No. 4,177,334, U.S. Pat. No. 4,250,138).
[0009] The process is generally applied to molding of PTFE alone or
PTFE containing several wt. % of a filler, and is not applied to
molding wherein a filler is the main component and PTFE is used
merely as a mechanical supplement. Namely, since the filler such as
graphite, glass fiber or carbon fiber is not likely to undergo
plastic deformation, in the case where it is mixed with PTFE and
molded, the extrusion pressure tends to be high, and PTFE is highly
deformed. Therefore, there is a problem that the obtained extruded
product is fragile and has low strength.
SUMMARY OF THE INVENTION
[0010] The present invention has been made to overcome the problems
of the prior art, and it is an object of the present invention to
provide an electrode having a shape of thin film sheet and having
high strength and low resistance, and a process for producing it,
thereby to provide an electric double layer capacitor having a high
capacitance density and small internal resistance, particularly an
electric double layer capacitor suitable for use wherein a large
current is required with a high capacitance.
[0011] The present invention provides a process for producing an
electrode for an electric double layer capacitor, which comprises
extruding a mixture comprising a carbonaceous material, PTFE and a
processing aid by paste extrusion, and rolling the obtained
extruded product by rolling rolls to form it into a sheet
shape.
[0012] Further, the present invention provides an electrode for an
electric double layer capacitor, which is a formed product of sheet
shape prepared by forming a carbonaceous material by means of PTFE
as a binder and which has a thickness of from 0.005 to 0.25 mm, a
porosity of from 50 to 80% and the tensile strength in one
direction is at least 1.5 kg/cm.sup.2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The electric double layer capacitor using the electrode of
the present invention is based on a principle to form an electric
double layer at the interface between an electrolytic solution and
a carbonaceous material which is a material for an electrode, and
to store electric charge in the electric double layer. As the
carbonaceous material, a powder of e.g. activated carbon, polyacene
or carbon black, which has a specific surface area of from 200 to
3500 m.sup.2/g, is preferred. Further, a fiber or a powder of e.g.
carbon fiber, carbon whisker or graphite can also be preferably
used, so long as its specific surface area is from 200 to 3500
m.sup.2/g. As the activated carbon, a phenol type, a rayon type, an
acryl type, a pitch type or a coconut shell type may be used. The
particle size of the activated carbon is preferably from 0.1 to 100
.mu.m, particularly preferably from 1 to 20 .mu.m, whereby it is
easy to form the electrode into a thin film sheet, and the
capacitance density can be made high.
[0014] It is also preferred to use carbon black in admixture with
another carbonaceous material, as a conductive material. In the
case of using it as a conductive material, the particle size of the
carbon black is preferably from 0.001 to 1 .mu.m, particularly
preferably from 0.01 to 0.5 .mu.m, thereby the resistance of the
electrode can be made low, even when it is contained in the
electrode in a small amount. Further, the specific surface area of
the carbon black as a conductive material, is preferably from 200
to 1500 m.sup.2/g, particularly preferably from 500 to 1200
m.sup.2/g. An electrode comprising the carbon black as a conductive
material, activated carbon having a specific surface area of from
200 to 3500 m.sup.2/g and a particle size of from 0.1 to 100 .mu.m,
and PTFE, is preferred since the internal resistance can be kept
low, and the capacitance can be kept high.
[0015] PTFE of the present invention includes not only a
homopolymer of tetrafluoroethylene but a copolymer obtained by
adding at most 0.5 mol % of another monomer to tetrafluoroethylene,
followed by copolymerization. When another monomer is at most 0.5
mol %, the melt fluidity is not given to PTFE, and it is possible
to form the copolymer into fibers to prepare an electrode sheet
having high strength and low resistance, just like the homopolymer
of tetrafluoroethylene. As another monomer, hexafluoropropylene,
chlorotrifluoroethylene, perfluoro(alkyl vinyl ether),
trifluoroethylene or (perfluoroalkyl)ethylene may, for example, be
mentioned.
[0016] PTFE is a liquid or a gel when it is of low molecular
weight, and in such a state, it can hardly be formed into fibers.
Therefore, it is preferred that PTFE contains at least 50 wt. % of
a polymer having a molecular weight of at least 1.times.10.sup.6 as
calculated from the standard specific gravity. Further, PTFE
obtained by emulsion polymerization is preferred since it is easily
formed into fibers.
[0017] The processing aid of the present invention is added so that
PTFE is suitably formed into fibers and undergoes plastic
deformation. It is not particularly limited so long as it is a
liquid capable of wetting PTFE easily, and being removed easily
from the formed electrode sheet. Specifically, ethanol, methanol,
2-propanol, lamp oil, solvent naphtha, white naphtha, ethylene
glycol, propylene glycol, dipropylene glycol or glycerol may, for
example, be mentioned. Further, an aqueous dispersion of PTFE may
be used as the processing aid, and it may be used alone or in
combination with another processing aid.
[0018] In the process for producing an electrode of the present
invention, PTFE, the carbonaceous material and the processing aid
are mixed so that PTFE is contained in the electrode in an amount
of preferably from 1 to 50 wt. %, particularly preferably from 5 to
30 wt. %, to the carbonaceous material. Sine PTFE is contained in
the electrode sheet as a binder to keep the shape of the electrode
sheet, if PTFE is less than 1 wt. %, the strength tends to be low,
and if PTFE is more than 50 wt. %, the internal resistance of the
electrode tends to increase.
[0019] In the process for producing an electrode of the present
invention, the processing aid may be added after mixing the
carbonaceous material and PTFE or when mixing them. The mixture
comprising the carbonaceous material, PTFE and the processing aid
may be a granulated product, but it does not influence the paste
extrusion. It is preferred to add from 20 to 200 wt. %,
particularly from 30 to 80 wt. %, of the processing aid to the
carbonaceous material. If the processing aid is less than 20 wt. %,
the fluidity for extrusion tends to be inadequate, and the
extrusion tends to be difficult. If the processing aid is more than
200 wt. %, the pressure for extrusion does not increase, PTFE tends
to be not adequately formed into fibers, and the processing aid is
likely to exude during the extrusion.
[0020] The mixture comprising the carbonaceous material, PTFE and
the processing aid is preliminarily molded, put into an extrusion
device, extruded by paste extrusion and formed into a formed
product of rod shape, sheet shape or tube shape. The extrusion
drawing ratio in the paste extrusion (a value obtained by dividing
a cross-sectional area of the cylinder mold in which a
preliminarily formed product is put, by a cross-sectional area of
the nozzle for extruding the formed product) is preferably from 5
to 500, more preferably from 10 to 100, particularly preferably
from 20 to 60. If it is less than 5, the extruded product tends to
be too soft to maintain its shape. If it is more than 500, the
extrusion becomes difficult, and the obtained extruded product
tends to be fragile.
[0021] In the present invention, after paste extrusion it is
possible to intermittently and sequentially supply the
preliminarily molded product of the mixture to a cylinder mold for
extrusion molding, under such a condition that the extruded product
remains in the nozzle, to continuously extrude it by extrusion to
obtain an elongate extruded product. By rolling the elongate
extruded product, an elongate electrode sheet can be obtained.
[0022] In the present invention, by rolling by rolling rolls
(hereinafter referred to as rolling), the extruded paste product is
formed into a sheet shape. The temperature of the rolling rolls is
preferably from 20 to 350.degree. C., particularly preferably from
60 to 120.degree. C. If the temperature of the rolling rolls is
lower than 20.degree. C., PTFE is not adequately formed into
fibers, and the sheet tends to be fragile. If the temperature of
the rolling rolls is higher than 350.degree. C., the processing aid
will significantly evaporate, and cracking or separation is likely
to result on the surface of the sheet.
[0023] With regard to the sheet formed by rolling, the processing
aid is required to be removed by drying. The processing aid may be
removed by drying after the sheet is formed by rolling. A part or
whole of the processing aid may be removed by drying during the
rolling. The drying temperature is preferably a temperature higher
than the boiling point of the processing aid and lower than the
melting point of PTFE. Further, it is possible to roll the dried
product of sheet shape or the semi-dried product of sheet shape
wherein the processing aid is partially removed, by rolling rolls
after stretching.
[0024] In the case of stretching, the stretching ratio is
preferably from 1.1 to 5.0 times, and the stretching is conducted
monoaxially or multiaxially. Further, the stretching may be
conducted before the drying step. By stretching, the forming of
PTFE into fibers is accelerated, and a thin film sheet having high
strength and low resistance can be obtained. The temperature during
stretching is preferably from 30 to 350.degree. C., particularly
preferably from 200 to 320.degree. C., whereby the forming of PTFE
into fibers can be more accelerated.
[0025] The electrode sheet obtained by the present invention can be
finished to be a thin film. The thickness of an electrode sheet of
an electric double layer capacitor is preferably thin, for the
purpose of increasing the capacitance density. However, if it is
too thin, the strength of the electrode sheet will be inadequate
and handling becomes difficult, when preparing an electric double
layer capacitor element by laminating or rolling up electrode
sheets with a separator interposed therebetween. Accordingly, it is
preferably from 0.005 to 0.25 mm, particularly preferably from 0.05
to 0.19 mm.
[0026] The porosity of an electrode sheet is preferably from 50 to
80%. If it is less than 50%, the capacitance density of the
electric double layer capacitor can not be made high, and if it is
more than 80%, the internal resistance will increase. The porosity
is calculated from the formula of (1-apparent density/true
density).times.100 (%).
[0027] The electrode sheet obtained by the present invention can be
used as an electrode as it is initially formed. However, it may be
used after baking, as the case requires. The baking may be complete
baking at a temperature higher than the melting point of PTFE or
incomplete baking at a temperature lower than the melting point of
PTFE.
[0028] By the process of the present invention, an extruded product
having an excellent shape keeping property can be obtained, wherein
PTFE is formed into fibers in the extrusion direction to have a
network structure by paste extrusion, and the carbonaceous material
is kept by the network structure of PTFE. With regard to the
strength of the electrode sheet, the tensile strength in one
direction is preferably at least 1.5 kg/cm.sup.2, particularly
preferably at least 2.0 kg/cm.sup.2. Generally, the direction is
the paste extrusion direction. In the present specification, the
tensile strength of an electrode is a value obtained by dividing
the maximum load, when the electrode sheet is dried for one hour at
a temperature of 250.degree. C., punched to a shape of a dumbbell
specimen of No. 1 as stipulated in JIS K6301, and subjected to a
tensile test at a pulling rate of 200 mm/min at an atmosphere
temperature of 25.+-.2.degree. C., by the cross-sectional area (the
thickness of the electrode sheet.times.the width of the parallel
parts).
[0029] Further, in the paste extrusion, the cross-sectional area in
the direction vertical to the extrusion direction of the extruded
product, is made gradually narrow from the cylinder mold so that
pressure is exerted to the desired nozzle for deformation.
Therefore, holes seen in the kneaded product are less likely to
remain. By rolling the extruded product, the forming of PTFE into
fibers is further accelerated. Accordingly, even when the amount of
PTFE is small, the electrode sheet formed into a thin film, is
excellent in crack resistance, break resistance and shape keeping
property, and has high strength.
[0030] Further, since PTFE is formed into fibers, and has a
three-dimensional network structure, the increase of the resistance
of the electrode by blending PTFE is small. Further, by stretching
the product of sheet shape, the forming of PTFE into fibers is
accelerated and the three-dimensional network structure spreads,
and the resistance of the electrode will further decrease. Further,
in the case where carbon black is added as a conductive material, a
high pressure is exerted on carbon black in both steps of paste
extrusion and rolling, whereby the electrode will have low
resistance by electrical connection even with a small amount of
carbon black.
EXAMPLES
EXAMPLE 1
[0031] Now, the present invention will be described in further
detail with reference to Examples. However, it should be understood
that the present invention is by no means restricted to such
specific Examples.
[0032] To a mixture comprising 80 wt. % of a phenol type high
purity activated carbon powder having a specific surface area of
1500 m.sup.2/g and an average particle size of 10 .mu.m, 10 wt. %
of carbon black having a specific surface area of 1270 m.sup.2/g
and an average particle size of 0.03 .mu.m, and 10 wt. % of PTFE
powder, ethanol was added in an amount of 60 wt. % based on the
carbonaceous material (activated carbon and carbon black), followed
by mixing. The mixture was preliminarily molded to a rectangular
parallelopiped shape, and paste extrusion was conducted by using a
nozzle having an extrusion drawing ratio of 40 and a rectangular
cross section. The obtained extruded product was rolled by rolling
rolls at a temperature of 80.degree. C., followed by drying at a
temperature of 250.degree. C. for 30 minutes. Ethanol was removed
and a sheet having a thickness of 120 .mu.m and a porosity of 66%
was obtained.
[0033] The sheet was dried at a temperature of 250.degree. C. for
one hour, punched into a shape of a dumbbell specimen of No. 1 form
as stipulated in JIS K6301, and subjected to a tensile test at a
pulling rate of 200 mm/min at an atmospheric temperature of
25.degree. C. to measure the maximum load. The measurement was
repeated three times, and the average value was taken as the
maximum load applied to the sheet. In order to measure the tensile
strength of the sheet in paste extrusion direction, the dumbbell
specimen was punched so that the longitudinal direction was the
paste extrusion direction. The tensile strength of the sheet
calculated from the value of the maximum load was 3.5
kg/cm.sup.2.
[0034] To one side of a pure aluminium foil of a rectangular shape
having a width of 4 cm and a height of 6 cm, and a thickness of 50
.mu.m, and having a lead terminal, an electrode sheet obtained by
punching the above sheet into an area of 4 cm.times.6 cm, was
bonded by means of a conductive adhesive, followed by heat curing
of the adhesive to obtain an electrode assembly. Two sheets of such
electrode assemblies were prepared, and the electrode sides of the
electrode assemblies were faced each other. A glass fiber separator
having a thickness of 40 .mu.m was interposed between the electrode
assemblies, which were then sandwiched between two glass plates
having a thickens of 2 mm, a width of 5 cm, and a height of 7 cm,
to obtain an element. The total thickness of the two electrode
assemblies and the separator was 0.39 mm.
[0035] As an electrolytic solution, a solution having 1.5 mol/l of
triethylmonomethylammonium tetrafluoroborate dissolved in propylene
carbonate, was used. The element was subjected to vacuum heating
for 3 hours at a temperature of 200.degree. C., to remove
impurities from the element, and then it was impregnated with the
electrolytic solution under vacuum and then accommodated in a
polypropylene rectangular bottomed cylindrical container, to obtain
an electric double layer capacitor. The direct current resistance
and the capacitance were measured at a current density of 20
mA/cm.sup.2, and the capacitance per volume (capacitance density)
and the resistance per volume were calculated. The results are
shown in Table 1.
EXAMPLE 2
[0036] A sheet was prepared in the same manner as in Example 1,
except that the thickness was made to be 80 .mu.m and the porosity
was made to be 64% by rolling. The tensile strength of the sheet in
the paste extrusion direction was measured in the same manner as in
Example 1, and found to be 2.0 kg/cm.sup.2. An electric double
layer capacitor was prepared in the same manner as in Example 1,
except that an electrode sheet obtained from the sheet was used,
and the same evaluation as in Example 1 was conducted. The results
are shown in Table 1.
EXAMPLE 3
[0037] A sheet was prepared in the same manner as in Example 1,
except that the thickness was made to be 150 .mu.m and the porosity
was made to be 69% by rolling. The tensile strength of the sheet in
the paste extrusion direction was measured in the same manner as in
Example 1, and found to be 3.1 kg/cm.sup.2. An electric double
layer capacitor was prepared in the same manner as in Example 1,
except that an electrode sheet obtained from the sheet was used,
and the same evaluation as in Example 1 was conducted. The results
are shown in Table 1.
EXAMPLE 4
[0038] A sheet was prepared in the same manner as in Example 1,
except that a sheet having a thickness of 150 .mu.m was obtained by
rolling, then the processing agent was dried, and then stretching
was conducted under a stretching ratio of 1.5 times at a
temperature of 300.degree. C. to make the thickness to be 110 .mu.m
and the porosity to be 69%. The tensile strength of the sheet in
the paste extrusion direction was measured in the same manner as in
Example 1 and found to be 2.8 kg/cm.sup.2. An electric double layer
capacitor was prepared in the same manner as in Example 1, except
that an electrode sheet obtained from the sheet was used, and the
same evaluation as in Example 1 was conducted. The results are
shown in Table 1.
EXAMPLE 5 (COMPARATIVE EXAMPLE)
[0039] A sheet was prepared in the same manner as in Example 1,
except that press forming by a press was carried out without
conducting paste extrusion, rolling was repeated 5 times to make
the thickness to be 250 .mu.m and the porosity to be 86%. The
tensile strength of the sheet in the stretching direction was
measured in the same manner as in Example 1, and found to be 1.3
kg/cm.sup.2. An electric double layer capacitor was prepared in the
same manner as in Example 1, except that an electrode sheet
obtained from the sheet was used, and the same evaluation as in
Example 1 was conducted. The results are shown in Table 1.
EXAMPLE 6 (COMPARATIVE EXAMPLE)
[0040] A sheet having a thickness of 150 .mu.m was obtained in the
same manner as in Example 1, except that press forming by a press
was carried out without conducting paste extrusion, and rolling was
repeated 10 times. However, many holes were formed in the sheet,
and the sheet could not be used as an electrode.
1 TABLE 1 Po- Capaci- ros- Internal Capaci- tance Resistance ity
resistance tance Volume density per volume (%) (.OMEGA.) (F)
(cm.sup.3) (F/cm.sup.3) (.OMEGA./cm.sup.3) Example 1 70 0.25 10.3
0.94 11.0 0.27 Example 2 64 0.26 8.2 1.06 9.7 0.30 Example 3 69
0.32 11.9 1.08 11.0 0.30 Example 4 69 0.24 10.1 0.94 10.7 0.26
Example 5 86 0.95 15.1 1.56 9.7 0.61
EFFECTS OF THE INVENTION
[0041] The electrode obtained by the process of the present
invention has high strength, since PTFE is formed into fibers to
have a three-dimensional network structure. It also has an
excellent crack resistance, break resistance and shape keeping
property, and low resistance, as an electrode of thin film sheet
shape. Particularly, when carbon black is contained as a conductive
material, the electrode becomes to have low resistance by
electrical connection, even if the amount of carbon black is small,
since high pressure is put on carbon black.
[0042] Therefore, the electric double layer capacitor comprising
the electrode of the present invention has small internal
resistance and large capacitance per volume.
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